1. Field of the Invention
This invention relates generally to testing for faults in cables and, more particularly, to apparatus and associated methodology for detecting and locating opens in cable shields or shield bonds.
2. Description of the Prior Art
Multipair, shielded cable is presently the predominant transmission medium utilized in the outside plant portion of a telephone system, that is, the portion serving to interconnect a central office with customer equipment. Continuity of the shield surrounding the multiple pairs composing a cable is important, particularly in the outside plant environment, for several reasons. First, from the viewpoint of safety to the customer or cable forces, a continuous shield provides a low resistance path to ground for lightning and power fault currents. Second, shield continuity reduces alternating current inductive interference. Poor cable shielding, caused in part by broken jumpers or bonds that bridge together contiguous segments of cable, allows unwanted signals to electromagnetically couple to the individual cable pairs and generate interference. Transmission quality is directly related to the effectiveness of shielding and noise mitigation requires a continuous shield.
Detecting and then locating shield faults occurring along a cable route can be difficult, time-consuming and expensive, especially if the facilities are buried or underground. To underscore the magnitude of the problem, the growth of buried plant over the past ten years has been three-fold; this growth emphasizes the need for equipment and a methodology capable of effectively locating shield faults such as open bonds. Since it is also well-known that defective bonds occur in significant numbers in both aboveground and cross-connect pedestals as well as in buried or underground sections, any newly devised technique must be quite versatile in capability.
To detect and locate open bonds in cables, various techniques and equipment are used that achieve varying degrees of success. Each method, however, has at least one major shortcoming.
U.S. Pat. No. 3,792,350, issued to F. C. Bossler et al, is representative of prior art techniques and devices categorized as tone tracers. The testing arrangement of Bossler et al employs: a signal source connected between shield and ground at an appropriate access point along the cable; and a portable probe to measure electromagnetic field components in two transverse directions at ground level near the anticipated fault. Certain preparatory steps are required in order to mitigate distortion of the field components; these steps include grounding the shield on the side of the open opposite the source and removing all other grounds. This latter step is unduly burdensome in many applications wherein multiple grounds are utilized to reduce inductive interference.
Other types of tone tracers can function as open bond locators if the bond is also faulted to ground with a resistance of approximately 500,000 ohms or less and no connections to power neutral are present.
Many of the limitations faced by these tracer methods are not applicable to approaches employing time domain reflectometry (TDR) techniques; however, conventional TDR also has deficiencies. An improved TDR methodology and associated apparatus, which are the subject matter of this inventio, mitigate these deficiencies.
Representative of prior art TDR approaches is an overview article entitled "Locating Cable Faults," by C. A. Maloney, published in the IEEE Transactions on Industrial Applications, July/August, 1973. The portion on pages 387 and 388 relating to "pulse reflection" is basically the conventional TDR approach. Time domain reflectometers differ from tracer sets and the like in that they operate in the time domain and provide the electrical distance to the fault as the result of measuring a pulse reflected from the fault. Reflectometers utilize the principle that a cable fault involves an impedance discontinuity characterized by a reflection parameter, so that a portion of an incident signal is reflected and returned to the source. The voltage returned is a function of the incident voltage weighted by the reflection parameter.
Reflectometers have been used to detect and locate conductor faults like opens, shorts and crosses as well as water and load coils. However, as open bond locators, conventional reflectometers have achieved limited success. A major part of the problem involves the TDR fault circuit configuration. One conventional use of the reflectometer as an open bond locator is to connect the instrument between the shield and ground at an access point. This shield-ground circuit may have severe distance limitations in many applications, particularly those involving multiple connections to power neutral. These multiple connections provide a low impedance path to ground resulting in high signal attenuation.
This problem may be alleviated by utilizing a shield-pair circuit to launch the incident voltage. This configuration does not have the attenuation problems associated with the shield-ground circuit. However, variations in the separation between the pair and the shield cause impedance fluctuations that produce random waviness, which appears as base-line clutter on a visual display of incident and reflected voltages. Although clutter decreases with distance, the interpretation of a returned voltage that falls within the clutter is difficult. In addition, dispersion, the unequal attenuation of pulse frequency components, can contribute to the already difficult task of fault identification by transforming the transmitted signal, usually a pulse, from a readily recognizable shape to a smeared, distorted shape which is not easily recognized.